The present invention relates to radio communication systems. More particularly, and not by way of limitation, the present invention is directed to a method and apparatus in a radio receiver for canceling multi-user interference in a single-carrier radio communication system.
In the reverse link of single-carrier radio communication systems such as Code Division Multiple Access (CDMA) systems, all users share the same frequency and time resources. Each user is differentiated by a unique long Pseudo-random Noise (PN) sequence. Since the user codes are not perfectly orthogonal, each user causes interference to all the other in-cell users. Hence the performance (for example capacity) is interference limited. An interference-canceling scheme can enhance the performance of such systems.
There are a number of canceling schemes in the literature, which describe ways to remove the multi-user interference. Such schemes include, for example, Maximum Likelihood Detection (MLD) interference cancellation. Even though the performance of MLD is quite good in terms of Bit Error Rate (BER) or capacity, the complexity increases exponentially as the number of users increases. With a large number of users, MLD becomes highly impractical. On the other hand, filter-based techniques such as Matched Filter (MF) and Minimum Mean Square Error (MMSE) are less complex to implement but their performance is sub-optimal, especially at low signal-to-interference-plus-noise ratio (SINR).